# Eli5 . The difference between the decibel weights such as A B C Z

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Eli5 . The difference between the decibel weights such as A B C Z

In: Physics

All of the decibel weights are designed to apply some type of bias to some frequencies and ignore others.

For instance A is trying to bias frequencies that humans hear and ignore other lower and higher pitches that humans don’t hear because if you don’t hear it it’s not exactly going to factor into the sound level you’re experiencing and therefore the measurement should reflect the “filtering” that you’re hearing range applies.

Human hearing is not a flat frequency response: between the highest and lowest sounds we can ever hear, we hear some frequencies louder than others.

Additionally, the difference in our perception changes depending on how loud the signals are. Described in the [equal loudness curves](https://en.m.wikipedia.org/wiki/Equal-loudness_contour). Eg if you listen to a 1kHz tone at 40dB(SPL) you’d need a 100Hz tone to be 60dB(SPL) for it to sound the same loudness

A weighting is intended to take into account the frequency/level response of human hearing to moderate level sounds.

Z weighting is a flat frequency response. Rather than just saying it’s “flat” it defines that it’s flat between 10Hz and 20kHz within +/- 1.5dB.

C weighting is somewhat flat, but rolls off bass and treble that humans can hear (-3dB at 31.5Hz and 8kHz). This is ideal for measuring loud sounds, eg over 100dB(SPL).

B weighting is old and not used any more